To Marylin and Brenda:We are indeed extremelygrateful for your patienceand your strong,unwavering support.

AcknowledgmentWe are extremely grateful to our friend and colleague ProfessorMartin Schreiber for his critical review of the entire book and theseveral insightful comments he provided. Martin, you are truly agood man.

vii

PrefaceAbout 6 years have passed between this, the fifth edition of Fluid, Electrolyte, and Acid–Base Physiology, and the fourth edition. For this edition, Professor Kamel S. Kamel has taken the role of lead author, whileProfessor Marc Goldstein, because of other commitments and timeconstraints, has decided not to participate.Our initial intention with this edition was to provide limitedupdates of a few chapters. We ended up, however, extensively revisingthe book, so that it is almost entirely rewritten. Although the effortwas substantial and the time commitment was much more than weanticipated, we could not be more proud of the product. In this fifthedition of Fluid, Electrolyte, and Acid–Base Physiology, we have triedto provide a comprehensive, go-to guide to the diagnosis and management of fluid-electrolyte and acid–base disorders. The book aimsto move from basic physiology to pathophysiology to practical clinical guidance, taking into account new discoveries and new insightsinto fluid-electrolyte and acid–base physiology, as well as new optionsavailable for treatment. We emphasize principles of metabolic regulation and biochemistry to promote an in-depth understanding ofmetabolic acid–base disorders. We also emphasize integrative, wholebody physiology to provide a more in-depth understanding of thepathophysiology of fluid, electrolyte, and acid–base disorders. Thestyle of the book, which we believe has been appealing to readers, hasnot changed. As in previous editions, we have attempted to provideinformation in an easy-to-understand way, with emphasis on how toapply the information to clinical practice, supported by numerousdiagrams, flow charts, and tables. To engage and challenge the reader,we have included several clinical cases and questions throughout eachof the chapters in the book.We believe that this fifth edition of Fluid, Electrolyte, and Acid–BasePhysiology will provide a useful resource to learners at different levels,from medical students to postgraduate trainees, and to practitionerssuch as general internists and specialists with an interest in the area offluid-electrolyte and acid–base disorders.

viii

Interconversion of UnitsBecause some readers will be more familiar with the InternationalSystem of Units (SI units) and others will prefer the conventional unitsused in the United States, we provide the following conversion table.To convert units, multiply the reported value by the appropriate conversion factor.

Principles of Acid–BasePhysiologyIntroduction...................................................................................................... 4Objectives........................................................................................................... 4P A R T A CHEMISTRY OF H+ IONS....................................................................... 5H+ ions and the regeneration of ATP..................................................... 5Concentration of H+ ions............................................................................. 6P A R T B DAILY BALANCE OF H+ IONS............................................................. 7Production and removal of H+ ions........................................................ 7Buffering of H+ ions....................................................................................11Role of the kidney in acid–base balance..............................................15Urine pH and kidney stone formation................................................25P A R T C INTEGRATIVE PHYSIOLOGY.............................................................27Why is the normal blood pH 7.40?.......................................................27Metabolic buffering of H+ ions during a sprint...............................28Discussion of questions.............................................................................29

3

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acid–base

Introduction

ABBREVIATIONBBS, bicarbonate buffer systemDEFINITIONS•Acids are compounds that arecapable of donating H+ ions;when an acid (HA) dissociates, ityields an H+ ion and its conjugatebase or anion (A−).•Bases are compounds that arecapable of accepting H+ ions.•Valence is the net electrical chargeon a compound or an element.HA ⇌ H + + A −

ACIDEMIA VERSUS ACIDOSIS•Acidemia describes an increasedconcentration of H+ ions in plasma.•Acidosis is a process in whichthere is an addition of H+ ionsto the body; this may or may notcause acidemia.ACID–BASE TERMS•Concentration of H+ ions: Thenormal value in plasma is 40 ± 2nmol/L, which is 0.000040mmol/L.•pH is the negative logarithm ofthe [H+] in mol/L, its normal valuein plasma is 7.40 ± 0.02.−•HCO3 ions: the conjugate baseof carbonic acid is the “H+ ionremover” of the BBS; its concentration in plasma is close to25 mmol/L, but there are largefluctuations throughout the day(22 to 31 mmol/L).•PCO2: The major carbon wasteproduct of fuel oxidation is carbondioxide. Its concentration isreflected by its partial pressure(PCO2). The normal arterial PCO2is 40 ± 2 mm Hg. The PCO2 inblood-draining skeletal muscles is∼6 mm Hg greater than the arterial PCO2 at rest.

Our goal in this chapter is to describe the physiology of hydrogen ions (H+)and how acid–base balance is achieved. From a chemical perspective, H+is the smallest ion (atomic weight 1) and its concentration in body fluidsis tiny (a million-fold lower than that of its major partner, HCO3−). Never­theless, H+ ions are extremely powerful because they are intimately involvedin the capture of energy from oxidation of fuels by driving regenerationof adenosine triphosphate (ATP4−). In this context, the electrical charge onthe protons is far more important than their chemical concentration.The concentration of H+ ions in body fluids must be maintained in avery narrow range. If their concentration rises, H+ ions will bind to intracellular proteins, and this changes their charge, shape, and possibly their functions, with possible dire consequences. Hence, a system is needed to removeH+ ions, even if their concentration is not appreciably elevated. This function is achieved by the bicarbonate buffer system (BBS). The special featurethat allows the BBS to function as an effective buffer is that a low PCO2drives the reaction of H+ ions with HCO3− anions (see Eqn 1). Because asmall increase in H+ ion concentration in plasma stimulates the respiratorycenter and causes hyperventilation, the concentration of CO2 in each literof alveolar air and hence in the arterial blood will be lower. Nevertheless,as we stress throughout this chapter, because the bulk of the BBS is in theintracellular fluid and the interstitial space of skeletal muscles, a low PCO2in their capillary blood is required to ensure the safe removal of H+ ions.Removal of H+ ions by the BBS leads to a deficit of HCO3− ions. Accordingly, one must have another system that adds new HCO3− ions to thebody as long as acidemia persists. This task is achieved by the kidneys, inthe metabolic process of excretion of ammonium ions (NH4+ ) in the urine.A high rate of excretion of NH4+ ions must be achieved while maintaining a urine pH that is close to 6.0 to avoid precipitation of uric acid. Basebalance is maintained by excreting an alkali load in the urine as a familyof organic anions rather than HCO3− ions. This avoids having a high urinepH and the risk of precipitation of calcium phosphate in the luminal fluid.

H + + HCO3−

CO2 + H2 O

(1)

OBJECTIVES

nTo describe the major processes that lead to acid and base balance.

Acid Balance 1.Production of acids: H+ ions are produced in a metabolic processwhen all of their products have a greater anionic charge than allof their substrates. 2.Buffering of H+ ions: This should minimize H+ ion binding toproteins in vital organs (i.e., the brain and the heart). To do so,−H+ ions must react with HCO3− ions. The vast majority of HCO3ions in the body is in the interstitial and intracellular compartments of skeletal muscle. The key to achieving this function is tohave a low PCO2 in the capillaries of skeletal muscle. 3.Kidneys add new HCO 3− ions to the body: This occurs primarily when NH4+ ions are excreted in the urine.

Base Balance 1.Input of alkali: This occurs primarily when fruit and vegetablesare ingested because they contain the K+ salts of organic acidsthat are metabolized to yield HCO3− anions. 2.Elimination of alkali: This is achieved in a two-step process: (1)the alkali load stimulates the production of endogenous organic

1 : principles of acid–base physiology−acids (e.g., citric acid), the H+ ions of which eliminate HCO3anions, and (2) the kidneys excrete organic anions (e.g., citrateanions) with K+ ions in the urine.nTo emphasize that acid–base balance is achieved whilemaintaining the urine pH close to 6.0. This minimizes therisk of forming uric acid precipitate if the urine pH wereacidic (pK = 5.3), or calcium phosphate precipitate if theurine pH were alkaline (pK = 6.8). In addition, eliminatingalkali via the excretion of organic anions (e.g., citrate anions)lowers the concentration of ionized calcium in the urine.

PART A

CHEMISTRY OF H+ IONS

H+ IONS AND THE REGENERATION OF ATPThree important steps constitute the metabolic process for the regeneration of ATP (called coupled oxidative phosphorylation); this involvesH+ ions in a major way. First, the energy needed to perform biological work in the cytosol of cells (e,g., ion pumping by Na-K-ATPase) isprovided when the terminal high-energy phosphate bond in ATP4- ishydrolyzed. This converts ATP4- to adenosine diphosphate (ADP3−),divalent inorganic phosphate ( HPO4 2 − ) ions, and H+ ion. Second,ADP enters the mitochondria on the adenine nucleotide translocator,while ATP exits. HPO4 2 − ions and H+ ions enter mitochondria by asymporter. Third, oxidation of the reduced form nicotinamide adeninedinucleotide (NADH, H+) produces nicotinamide adenine dinucleotide (NAD+) and two electrons. This represents the first step in theelectron transport chain. Flow of these electrons through coenzyme Qand ultimately cytochrome C releases the energy that is used to pumpH+ ions from the mitochondrial matrix through the inner mitochondrial membrane. This creates a very large electrical driving force (∼150mV) and a smaller chemical driving force for H+ ion re-entry. This

Hϩ

Hϩ

NADH ϩ HϩNADϩMatrixADP + Pi

ATP

Figure 1-1 H+ Ions and the Regeneration of ATP. The horizontal structure represents the inner mitochondrial membrane with its inner and outer bilayers.The dashed line at the top represents the outer mitochondrial membrane. Oxidation of the reduced form of nicotinamide adenine dinucleotide (NADH, H+)produces NAD+ and two electrons. Flow of these electrons through the electrontransport chain releases the energy that is used to pump H+ ions from themitochondrial matrix through the inner mitochondrial membrane. This createsa very large electricochemical driving force for H+ ion re-entry. This energy isrecaptured as H+ ions flow through the H+ ion channel portion of the H+adenosine triphosphate (ATP) synthase in the inner mitochondrial membrane,which is coupled to ATP regeneration from ADP and inorganic phosphate (Pi).ADP, Adenosine diphosphate.

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acid–base

energy is recaptured as H+ ions flow through the H+ ion channel portion of the H+-ATP synthase in the inner mitochondrial membrane,which is coupled (linked) to ATP4- regeneration provided that ADP3and HPO4 2 − are available inside these mitochondria (Figure 1-1).Hence, availability of ADP in the mitochondria sets an upper limit onthe rate of coupled oxidative phosphorylation (see margin note).

Uncoupling of Oxidative Phosphorylation

ATP/ADP TURNOVER•It is important to appreciate thatthe actual concentration of ATPin cells is small (∼5 mmol/L)and that of ADP is extremely tiny(∼0.02 mmol/L), but their rate ofturnover is enormous.•The weight of ATP in the brain isjust a few grams (concentrationof ATP 0.005 mol/L, molecularweight ∼700 g/mol, brain weightin adult of about 1.5 kg— 80% ofwhich is intracellular fluid [ICF]).•The brain consumes close to3 mmol of O2 per minute or4.5 mol of O2 per day. Because~6 mol of ATP are formed permole of O2 consumed, the brainregenerates 27 mol of ATP per day(4.5 mol of O2 × 6 ATP/O2). Hence,the daily turnover of ATP in thebrain is almost 20 kg (27 mol × molwt ∼700 g/1000 = 18.9 kg).BENEFIT OF H+ ION BINDINGTO HEMOGLOBIN•When H+ ions bind to hemoglobinin systemic capillaries, hemoglobin can off load oxygen (O2) at ahigher PO2, which improves thediffusion of O2 into cells.•In contrast, when H+ ions dissociate from hemoglobin in thecapillaries in the lungs (drivenby a higher PO2), this leads to agreater uptake of O2 from alveolarair for a given alveolar PO2.AMOUNT OF H+ IONS IN THEBODY•ECF: 15 L × 40 nmol/L = 600 nmol•ICF: 30 L × 80 nmol/L = 2400 nmol

This limitation by availability of ADP3- (rate of biological work) on therate of fuel oxidation can be bypassed if oxidation of more fuel thanwhat is needed to regenerate ATP4- is advantageous. This is achievedby uncoupling of oxidative phosphorylation. In this process, H+ ionsre-enter the mitochondrial matrix by a different H+ ion channel, onethat is not linked to the conversion of ADP3- to ATP4-.

CONCENTRATION OF H+ IONSThe concentration of H+ ions in all body compartments must be maintained at a very low level. This is because H+ ions bind very avidly tohistidine residues in proteins. Binding of H+ ions to proteins changestheir charge to a more positive valence, which might alter their shape,and possibly their functions. Because most proteins are enzymes,transporters, contractile elements, and structural compounds, a changein their functions could pose a major threat to survival. Nevertheless, there are examples when this binding of H+ ions to proteins hasimportant biologic functions (see margin note).The concentration of H+ ions in body fluids is exceedingly tiny (inthe nmol/L range) and, moreover, is maintained within a very narrowrange. In the extracellular fluid (ECF) compartment, the concentration of H+ ions is 40 ± 2 nmol/L, while in the ICF compartment, theconcentration of H+ ions is ∼80 nmol/L. In fact, the concentration oftheir partner, HCO3− ions, in the ECF compartment (∼25 mmol/L), isalmost one million-fold higher than that of H+ ions.This is impressive because an enormous quantity of H+ ions is produced and removed by metabolism each day relative to the amount ofH+ ions in the body (see margin note). In more detail, acids are obligatory intermediates of carbohydrate, fat, and protein metabolism. Forexample, because adults typically consume and oxidize about 270 g(1500 mmol) of glucose per day, at least 3000 mmol (3,000,000,000nmol) of H+ ions are produced as pyruvic and/or L-lactic acids in glycolysis when work is performed and ATP4− is converted to ADP3−.The complete oxidation of pyruvate/L-lactate anions to CO2 and H2Oremoves the H+ ions almost as quickly as they are formed. In an adulteating a typical Western diet, a net of ~70 mmol (70,000,000 nmol)of H+ ions are added daily to the body. Hence, small discrepanciesbetween the rates of formation versus removal of H+ ions, if sustained,can result in major changes in concentration of H+ ions. This impliesthat there are very effective control mechanisms that minimize fluctuations in concentration of H+ ions in body fluids.QUESTIONS(See Part C for discussion of questions)1-1 In certain locations in the body, H+ ions remain free and are not bound.What is the advantage in having such a high concentration of H+ ions?1-2 What is the rationale for the statement, “In biology only weak acids kill”?

1 : principles of acid–base physiology

PART B

DAILY BALANCE OF H + IONS

PRODUCTION AND REMOVAL OF H+ IONS

•H+ ion production: H+ ions are produced when neutral compounds are converted to anions.•H+ ion removal: H+ ions are removed when anions are converted to neutral products.

+To determine whether H ions are produced or removed duringmetabolism, we use a “metabolic process” analysis. A metabolicprocess is made up of a series of metabolic pathways that carry outa specific function; these pathways may be located in more thanone organ. To establish the balance for H+ ions in a metabolic process, one needs only examine the valences of all of its substratesand products, while ignoring all intermediates (see Chapter 5 formore details). If the sum of all of these valences is equal, there is nonet production or removal of H+ ions. When the products of a metabolic process have a greater anionic charge than its substrates, H+ions are produced (e.g., incomplete oxidation of the major energyfuels, carbohydrates, and fats). Conversely, when the products of ametabolic process have a lesser anionic charge than its substrates,H+ ions are removed.About 85% of kilocalories consumed, in a typical Western diet, arein the form of carbohydrates and fat. There is no net production ofH+ ions when glucose and triglycerides are completely oxidized toCO2 + H2O because the substrates and the end products of thesemetabolic processes are neutral compounds. There is a net H+ ionload, however, when complete oxidation of these fuels does not occur.L-Lactic acid accumulates during hypoxia, because its rate of production from glycolysis far exceeds its rate of removal via oxidation and/or gluconeogenesis. Ketoacids are produced during states of a net lackof insulin if their rate of production from metabolism of free fattyacids (triglycerides) in the liver exceeds their rate of removal by thebrain and the kidneys.The metabolism of certain dietary constituents leads to theaddition of H+ ions (e.g., proteins) or HCO3− ions (e.g., fruit andvegetables) to the body. A general overview of the components ofthe daily turnover of H+ ions is illustrated in Figure 1-2. Overall,one must examine balances for both acids and bases to have a trueassessment of H+ ion balance.

Acid BalanceOxidation of two classes of amino acids (cationic amino acids [e.g.,lysine, arginine] and sulfur-containing amino acids [e.g., cysteine,methionine]) yields an H+ ion load (Table 1-1). In contrast, H+ ionsare removed during the oxidation of anionic amino acids (e.g., glutamate, aspartate), because all the products of their oxidation are neutral compounds (urea, glucose, or CO2 + H2O). Because the numberof cationic and anionic amino acids is nearly equal in the amino acidmixture in beefsteak, the H+ ion load that causes a deficit of HCO3−ions is mainly from the metabolism of sulfur-containing amino acidsthat yield sulfuric acid (H2SO4).

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acid–base

Acid-Base Balance

Acid balanceProduction of H

Base balance

+

Production of HCO3Ϫ+

2 H + SO42Ϫ

Diet

+

2 K + 2 HCO3Ϫ

Diet

+

Removal of H2 H+ + 2 HCO3Ϫ

Removal of HCO3Ϫ2 CO2 + 2 H2O

+

Glucose

2 H + Citrate

2Ϫ

Excrete organic anions

Add “new” HCO3Ϫ

2 NH4+ + SO42Ϫ

Urine

Urine

+

2 K + Citrate 2Ϫ

Figure 1-2 Overview of the Daily Turnover of H+ Ions. Acid balance is shown on the left, and basebalance is shown on the right. There are three components to acid balance: (1) production of H+ions, (2) HCO3− ions remove this H+ ion load, and (3) the kidneys add new HCO3− ions to thebody when NH4+ ions are excreted in the urine. There are also three components to base balance:(1) the alkali load of the diet is converted to HCO3− ions in the liver, (2) organic acids are formedin the liver and their H+ ions remove HCO3− ions, and (3) excretion of these new organic anionsalong with the potassium (K+) ions from the diet in the urine.TABLE 1-1 H+ ION FORMATION OR REMOVAL IN METABOLIC

H2SO4H+ ions cannot be eliminated by metabolism of SO4 2 − anions to neutral end products (because no such pathway exists) or by being excretedbound to SO4 2 − anions in the urine (because of the low affinity ofSO4 2 − anions for H+ ions). Hence, these H+ ions must be titrated initially with HCO3− ions and, as a result, CO2 is formed. Acid balance isrestored when these SO4 2 − anions are excreted in the urine with anequivalent amount of NH4+ ions because new HCO3− ions are generatedin this process (Figure 1-3).

1 : principles of acid–base physiologyDiet

1

ECF

2

2 HCO3Ϫ

2 Hϩ

2 CO2 + 2 H2O

Sulfur-AA2 HCO3Ϫ

SO42ϪUrine

Glutamine

3

SO42Ϫ

2 NH4ϩ

2 NH4ϩ

Figure 1-3 H+ Ion Balance during the Metabolism of Sulfur-Containing Amino Acids. Renal events are represented in the large shaded area. When sulfurcontaining amino acids are converted to SO4 2 − anions, H+ ions are produced (site 1). H+ ions react with HCO3− ions, and this produces a deficit ofHCO3− ions in the body (site 2). To achieve H+ ion balance, new HCO3− ionsmust be regenerated. Metabolism of the amino acid glutamine in cells ofproximal tubules produces NH4+ ions and dicarboxylate anions. HCO3− ionsare added to the body when these anions are metabolized to a neutral endproduct and NH4+ ions are excreted in the urine with SO4 2 − anions (site 3).ECF, Extracellular fluid.CO2 + H2OK+

+RNA-P−

K+ +

H2 PO4−H+

HCO3−

2 K++P-Cr2−

2 K+ + H PO42−HCO3−

2 K+ + H PO42−HCO3−

K+H+K+ + H2 PO4−

HCO3−

CO2 + H2O

H+K+ + H2 PO4−

CO2 + H2OK+ + OA−

Figure 1-4 H+ Ion Balance during the Metabolism of Organic Phosphates. The upper rectanglerepresents the body, the lower, large shaded rectangle represents events in the kidney, the smallshaded rectangle represents excretion in the urine. The acid–base impact of the metabolic processinvolving phosphate depends on whether their metabolism resulted in the addition of the monovalent inorganic phosphate (H2 PO4−) or the divalent inorganic phosphate (HPO4 2 − ) to the body.As shown in the left panel of the figure, if H2 PO4− were added to the body and then excreted in theurine as H2 PO4−, there is no net loss or gain of HCO3− ions in this process. On the other hand, ifHPO4 2 − were added to the body, at a urine pH of ∼6 it will be excreted as H2 PO4−. Hence, a newHCO3− ion is generated in this process (right panel of the figure). To maintain acid–base balance inresponse to this alkali load, there is increased production of endogenous organic acids. Their H+ions remove these HCO3− ions, while their conjugate bases (organic anions [OA−]) are excreted inthe urine as K+ salts. P-Cr 2−, Phosphocreatine 2−; RNA-P, ribonucliec acid.

Dietary phosphateThe source of phosphate in the diet consists primarily of intracellularorganic phosphates (including energy storage compounds e.g., ATP4−and phosphocreatine2− in beefsteak, and nucleic acids [RNA, DNA])and phospholipids, which are primarily in organ meat (e.g., liver). Theaccompanying cation for both forms of intracellular organic phosphatesis primarily potassium (K+) ions. The acid–base impact of the metabolicprocess involving phosphate depends on whether their metabolismresulted in the addition of the monovalentinorganicphosphate (H2 PO4− )()2−or the divalent inorganic phosphate HPO4 to the body. In more detail,if H2 PO4− were added, because it has a pK of 6.8, close to one bound H+ion per H2 PO4− is released in the body at normal blood pH values (7.40)

(Figure 1-4). These H+ ions react with HCO3− ions, creating a deficit ofHCO3− ions in the body. To achieve H+ ion balance, new HCO3− ionsmust be regenerated. This occurs in two steps: (1) the kidney convertsCO2 + H2O to H+ ions + HCO3− ions and (2) these H+ are secreted and−bind to filtered HPO4 2 − anions. Thus, H2 PO4 is excreted when the urinepH is in the usual range (i.e., ∼6), while HCO3− ions are added to thebody. Hence, elimination of H+ ions produced during the metabolism oforganic phosphates to H2 PO4− does not require the excretion of NH4+ions. There is no net loss or gain of HCO3− ions in this process.On the other hand, if HPO4 2 − were added to the body, at a urine pHof ∼6, it will be excreted as H2 PO4−. Hence, new HCO3− ions are generated in this process. To maintain acid–base balance, one possible mechanism is increased production of endogenous organic acids in response tothis alkali load. Their H+ ions remove this HCO3− ion load, while theirconjugate bases are excreted in the urine as K+ salts (see Figure 1-4).

Base BalanceAll the emphasis so far has been on the production and removal of H+ions. The diet, however, also provides an alkali load that is produced during the metabolism of a variety of organic anions in fruit and vegetables(Figure 1-5). Although it would have been nice from a bookkeepingpoint of view to have these HCO3− ions titrate some of the H+ ion loadfrom H2SO4 produced from metabolism of sulfur-containing aminoacids, this occurs only to a minor extent. The advantage of not having thedietary alkali load titrate dietary acid load becomes evident when considered in the context of minimizing the risk of kidney stone formation.Dietary organic anions are first converted to HCO3− ions in the liver.This avoids having a potentially toxic anion enter the systemic circulation (e.g., citrate anions, which chelate ionized calcium in plasma). Inresponse to the alkali load, a variety of organic acids (e.g., citric acid) areproduced in the liver. The fate of their H+ ions is similar: the removalby HCO3− ions. To prevent the synthesis of HCO3− ions at a later time,the conjugate bases of these organic acids are made into end productsof metabolism by being excreted with K+ ions in the urine (see marginnote), and hence base balance is achieved. As discussed later, the pH ofcells of the proximal convoluted tubule (PCT) plays an important role indetermining the rate of excretion of citrate and other organic anions inthe urine. In fact, the rate of excretion of citrate in the urine is thought toprovide a window on pH in the cells of PCT (see margin note).Diet

ECF

1Kϩϩ

HCO3Ϫ

CO2

Hϩ

OAϪ

2Glucose

OAϪKϩOAϪUrine

3OAϪ

PHCO3 =less renal OAϪreabsorption

Figure 1-5 Overview of Base Balance. Base balance is achieved in threesteps. The first is the production of HCO3− ions from dietary K+ salts oforganic anions in the liver (site 1). This is followed by the production oforganic acids in the liver; their H+ ions titrate these HCO3− ions (site 2). Therenal component of the process is shown in the large shaded area (site 3).The organic anions are filtered and only partially reabsorbed by the kidney;hence, they are made into end products of metabolism by being excretedin the urine. ECF, Extracellular fluid.

1 : principles of acid–base physiology

From an integrative physiology point of view, the elimination ofdietary alkali in the form of organic anions has a number of advantagesin terms of minimizing the risk of kidney stone formation. In moredetail, it avoids the excretion of HCO3− ions, and hence the likelihood ofkidney stones that form when the urine pH is too high (e.g., CaHPO4).In addition, the elimination of this dietary alkali in the form of citrateanions lessens the likelihood of forming calcium-­containing kidneystones because citrate anions chelate ionized ­calcium in the urine.QUESTION(See Part C for discussion of questions)1-3 Does consumption of citrus fruit, which contains a large quantity ofcitric acid and its K+ salt, cause a net acid or a net alkali load?

BUFFERING OF H+ IONS

•The most important goal of buffering is to minimize the binding of H+ ions to intracellular proteins in vital organs (e.g., thebrain and the heart)

The traditional view of the buffering of H+ ions during metabolic acidosis is “proton-centered” (i.e., it focuses solely on diminishing theconcentration of H+ ions). It is based on the premise that H+ ions arevery dangerous; therefore, anything that minimizes a rise in their concentration is beneficial. An argument to support this view is that ahigh concentration of H+ ions may depress myocardial contractility.The evidence for this effect, however, is from experimental studies inanimals or isolated perfused hearts preparations. Furthermore, it isnot consistent with the very high cardiac output observed during asprint when the blood pH may be below 7.0. In addition, this view ofbuffering of H+ ions does not take into consideration the price to payto achieve this goal. In more detail, binding of H+ ions to proteins willchange their “ideal or native” valence (protein0) to become more cationic or less anionic (protein+) (see Eqn 2). This may alter their shapeand possibly their functions (as enzymes, transporters, contractile elements, or structural compounds), which may have deleterious effects.H+ + Protein0 → H·Protein+(2)We emphasize a different way to analyze buffering of an H+ ion loadand suggest that a “brain protein-centered” view of buffering of H+ ionsin the patient with metabolic acidosis may offer a better way to understand the pathophysiology, which has important implications for therapy. The major tenet of this view is that the role of buffering is not simplyto lower the concentration of H+ ions but to minimize the binding ofH+ ions to proteins in cells of vital organs (e.g., the brain and the heart).

Bicarbonate Buffer System

•H+ ions must by removed by the BBS to avoid their binding tointracellular proteins.•A low PCO2 is a prerequisite for optimal function of the BBS.

Even though at plasma pH of 7.4, the BBS is very far displacedfrom its pK (pH ∼6.1) and hence is not an ideal chemical buffer,

11

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acid–base(Tiny)ϩ

(Huge)

H + HCO3Ϫ

H2CO3

H2O + CO2

PTN0H•PTN ϩ

Figure 1-6 Buffer Systems. Proteins in cells have an “ideal charge” (depicted as PTN0). Binding of H+ ions to these proteins increases their netpositive charge (H·PTN+) and may compromise their functions. Hence, thekey principle is that new H+ ions must be removed by binding to HCO3−ions so that very few H+ ions can bind to proteins (PTN0) in cells. To forceH+ ions to bind to HCO3− ions, the PCO2 must fall in cells despite the factthat cells produce an enormous quantity of CO2.

nevertheless it is the most important physiologic buffer. This iscaused by the fact that it can remove H+ ions without requiring ahigh H+ ion concentration. As shown in Eqn 1, a low PCO2 “pulls”the BBS reaction to the right. As a result, the concentration ofH+ ions falls, which decreases the binding of H+ ions to proteins(­Figure 1-6). In addition, the BBS is capable of removing a largequantity of H+ ions because there is a large amount of HCO3− ionsin the body, ≈750 mmol in a 70 kg adult (see margin note).Which PCO2 is important for the bicarbonate buffer systemto function optimally?

ABBREVIATIONSEABV, effective arterial bloodvolume

•The arterial PCO2 reflects, but is not equal to, the PCO2 inbrain cells; it sets a minimum value for the PCO2 in capillariesof all other organs in the body.•The bulk of the BBS is in the interstitial space and in cells ofskeletal muscle, hence PCO2 in muscle capillary blood reflectsthe effectiveness of the BBS in removing an H+ ion load.

The process to lower the PCO2 begins with stimulation of the respiratory center in the brain. This is a most appropriate response becauseit ensures that the brain will always have an “ideal” PCO2 in its ECFand ICF compartments. In more detail, hyperventilation results ina lower arterial PCO2. Because the rate of production of CO2 in thebrain is relatively constant (i.e., its oxygen consumption does notvary appreciably and its blood flow is autoregulated), a lower arterial PCO2 will predictably result in a lower PCO2 in the ECF and ICFcompartments of the brain. Therefore, there is only a minimal bindingof H+ ions to intracellular proteins in the brain during metabolic acidosis, which decreases the possible detrimental effects on neuronalfunction. Accordingly, the arterial PCO2 reflects the PCO2 in braincells in the absence of a marked degree of contraction of the effectivearterial blood volume (EABV) during which the brain fails to autoregulate its rate of blood flow.The question, however, is whether a low arterial PCO2 is sufficientto ensure optimal function of the BBS in other organs. Because CO2diffuses rapidly, distances are short, and time is not a limiting factor, the PCO2 in capillaries is virtually identical to the PCO2 in cellsand in the interstitial compartment of the ECF in a given region.Therefore, it is the capillary PCO2 (rather than the arterial PCO2)that reveals whether the BBS has operated efficiently in removing aload of H+ ions (Table 1-2). Notwithstanding, the arterial PCO2 setsthe lower limit for the PCO2 in capillaries.

1 : principles of acid–base physiologyTABLE 1-2 THE BLOOD PCO2 AND ITS IMPLICATIONS FOR BRAIN

PROTEIN-CENTERED BUFFERING OF H+

SITE OF SAMPLING

BBS BUFFERING

FUNCTIONAL IMPLICATIONS

•Arterial PCO2

•Reflects the PCO2in brain if theblood flow rate is­autoregulated•Not really able todefine site of H+ ionbuffering•Reflects the PCO2 inskeletal muscle cellsand their interstitialcompartment

The capillary PCO2 is higher than the arterial PCO2 becausecells consume O2 and add CO2 to their capillary blood. The capillary PCO2 is influenced by the value of the arterial PCO2 and therate of addition of CO2 to capillary blood in individual organs. Forinstance, if most of the oxygen in each liter of blood delivered toa certain area is consumed, the PCO2 in its capillary blood willrise appreciably. There are two conditions in which most of theO2 delivered in a liter of blood is consumed: (1) a rise in the rateof metabolism without a change in the rate of blood flow, or (2) adecrease in the rate of blood flow with no change in the rate of O2consumption.Although the capillary PCO2 reveals whether the BBS has operatedefficiently, one cannot measure it directly. The venous PCO2, however,closely reflects the capillary PCO2 in its drainage bed. There is onecaveat—if an appreciable quantity of blood shunts from the arterialto the venous circulation and bypasses cells, this venous PCO2 doesnot reflect the PCO2 in the interstitial space and in cells in its drainage bed.The question now is which venous PCO2 should be measured toassess the effectiveness of the BBS. Because of its size, skeletal musclehas the largest content of HCO3− ions in the body in its cells and interstitial space. Therefore, in patients with metabolic acidosis, the PCO2should be measured in free-flowing brachial venous blood to assessthe effectiveness of the BBS.Failure of the bicarbonate buffer systemThe main cause of failure of the BBS in skeletal muscle is a verymarked decline in its blood supply—this is the case when metabolic acidosis is accompanied by a contracted EABV. Hence, whilethe arterial PCO2 may be low due to stimulation of the respiratorycenter by acidemia, the PCO2 in intracellular fluid and interstitialspace in muscle may not be low enough for effective buffering ofH+ ions by the BBS (Figure 1-7). As a result, the degree of acidemia may become more pronounced and more H+ ions may bind toproteins in the extracellular and intracellular fluids in other organs,including the brain. Notwithstanding, because of autoregulation ofcerebral blood flow, it is likely that the PCO2 in brain capillary blood

13

14

acid–baseNormal EABV

[H+]

CO2

H+ + HCO3−

HCO3− + H+

CO2PTN•H+

CO2

PTN0

PTN0

Muscle

CO2

BrainLow EABV

[H+]HCO3− + H+

CO2

↑CO2

PTN•H+

PTN0

Muscle

H+ + HCO3−PTN0

CO2

PTN•H+

CO2

Brain

Figure 1-7 Buffering of H+ Ions in the Brain in a Patient with a Contracted Effective ArterialBlood Volume (EABV). Buffering of H+ ions in a patient with a normal effective arterial bloodvolume and thereby a low venous PCO2 is depicted in the top portion of the figure. The vastmajority of H+ ion removal occurs by bicarbonate buffer system (BBS) in the interstitial spaceand in cells of skeletal muscles. Buffering of an H+ ion load in a patient with a contractedEABV and thereby a high venous PCO2 is depicted in the bottom portion of the figure. A highPCO2 prevents H+ ion removal by the BBS in muscles. As a result, the circulating H+ ion concentration rises, which increases the H+ ion burden for brain cells. Unless there is a severe degreeof contraction of the EABV and failure of auto-regulation of cerebral blood flow, the BBS in thebrain will continue to titrate much of this large H+ ion load. Because of the limited content ofHCO3− ions in the brain and because the brain receives a relatively larger proportion of bloodflow, there is a risk that more H+ ions will bind to proteins in the brain cells.

will change minimally unless there is a severe degree of contraction ofthe EABV and failure of autoregulation of cerebral blood flow. Hence,the BBS in the brain will continue to titrate much of this large H+ ionload. Considering, however, the limited content of HCO3− ions in thebrain, and that the brain receives a relatively larger proportion of thecardiac output, there is a risk that more H+ ions will bind to proteinsin the brain cells, further compromising their functions.In summary, patients with metabolic acidosis and a contractedEABV have a high PCO2 in venous blood draining skeletal muscle,and therefore they fail to titrate an H+ load with their BBS in skeletalmuscle. Hence, there is a much higher H+ ion burden in their braincells, with possible detrimental effects. At usual rates of blood flowand metabolic work at rest, brachial venous PCO2 is about 46 mmHg, which is ∼6 mm Hg greater than the arterial PCO2. If the bloodflow rate to the skeletal muscles declines because of a low EABV, thebrachial venous PCO2 will be increased to greater than 6 mm Hghigher than the arterial PCO2. Based on this analysis, it follows thatin patients with metabolic acidosis, the clinician should administerenough saline to increase the blood flow rate to muscle to restorethe usual brachial venous minus arterial PCO2 difference, i.e., backto ∼6 mm Hg.

1 : principles of acid–base physiology

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QUESTIONS(See Part C for discussion of questions)1-4 Why is the L-lactic acidosis that occurs during cardiogenic shock somuch more devastating than the L-lactic acidosis that occurs during a sprint if the Pl-lactate, arterial pH, and PHCO3 are identical?1-5 Th e heart extracts close to 70% of the oxygen from each liter of coronary artery blood. What conclusions can you draw about bufferingof H+ ions in the heart? Might there be advantages because of thishigh extraction of O2 per liter of blood flow?

ROLE OF THE KIDNEY IN ACID–BASE BALANCEThe kidneys must perform two tasks to maintain acid balance. First,the kidney must reabsorb virtually 100% of the filtered HCO3− ions;this is achieved primarily by H+ ion secretion in the PCT. Second, thekidneys must add new HCO3− ions to the body to replace what is lostin buffering of an added acid load; this is achieved principally in themetabolic process that ends by excreting NH4+ ions in the urine.

Reabsorption of Filtered HCO3– Ions

•The kidneys must prevent the excretion of the very large quantity of filtered HCO3− ions. In this process there is no additionof new HCO3− ions to the body.

It is important to recognize that a huge amount of HCO3− isfiltered and reabsorbed each day (GFR of 180 L/day × PHCO325 mmol/L = 4500 mmol). The bulk of filtered HCO3− ions (approximately80% [∼3600 mmol/day] [see margin note]) is reabsorbed by the PCT.

PERCENT OF FILTERED HCO3−REABSORBED IN PCT•This is a minimum estimate ofthe percent of the filtered HCO3−ions that is reabsorbed in PCT.This is because it is based ondata from micropuncture studiesin rats. The site of micropuncture, the last accessible part ofthe PCT on the surface pf thecortex, is, however, not the end ofthe PCT.

Reabsorption of NaHCO3 in the proximal convoluted tubuleReabsorption of HCO3− ions in PCT occurs in an indirect fashion viaH+ ion secretion. In this process, the PCT reclaims the vast majorityof filtered HCO3− ions, but there is no generation of new HCO3− ions.Nevertheless, if this process were to fail, there will be a loss of NaHCO3in the urine and the development of metabolic acidosis (this disorderis called proximal renal tubular acidosis; it is discussed in Chapter 4).The process of HCO3− ion reabsorption in PCT has four interconnected steps (Figure 1-8):1. H+ ion secretion into the luminal fluid. This is largely mediated by the Na+/H+ exchanger-3 (NHE-3) in theapical membrane of PCT cells. This is an electroneutral exchangerbecause for every Na+ ion reabsorbed, one H+ ion is secreted intoits lumen. The driving force to reabsorb Na+ ions by NHE-3 is provided by the very low concentration of Na+ ions inside PCT cells,because of the active transport of Na+ ions out of cells by Na-KATPase in their basolateral membrane.2. Secreted H+ ions combine with HCO3− ions in the lumen to form H2CO3. H2CO3 dissociates into CO2 and H2O. This dissociation reactionoccurs virtually as soon as H2CO3 is formed because it is catalyzedby the enzyme carbonic anhydrase IV (CAIV), an isoform of carbonic anhydrase that is bound to the brush boarder of PCT cells.CO2 that is formed in the lumen crosses the apical membrane andenters the PCT cells (see margin note).

ENTRY OF CO2 INTO PCT CELLS•This was thought to occur bydiffusion of CO2 through the lipidbilayer of the apical membrane ofPCT cells.•There are data to suggest thatentry of CO2 is via the luminalwater channel, AQP1, which canalso behave as a gas channel.

16

acid–base

Naϩ

Naϩ

NHE-3

HCO3Ϫ

NBCe1Hϩ

Hϩ

(HCO3Ϫ)3

H2CO3

H2CO3

CAIVCAII

CO2ϩH2OH2O

CO2

AQP1

Figure 1-8 Reabsorption of NaHCO3 in the Proximal Convoluted Tubule.The components of the process of indirect reabsorption of NaHCO3 areshown in the figure. H+ ion secretion is largely via Na+/H+ exchanger3 (NHE-3). HCO3− ions exit the cell via an electrogenic Na-bicarbonatecotransporter (NBCe1). This process requires a luminal carbonic anhydrase (CA)IV and intracellular CAII. CO2 that is formed in the lumen entersthe cell likely via a luminal aquaporin 1 water channel (AQP1).

3. I nside the cell, CO2 and H2O recombine to form H2CO3. Another isoform of carbonic anhydrase, carbonic anhydraseII (CAII) is present inside cells; it accelerates the dissociation ofH2CO3 into H+ ions and HCO3− ions. While H+ ions are secretedinto the luminal fluid, HCO3− ions exit the cell across the basolateral membrane, completing the process of indirect reabsorption ofHCO3− ions.4. HCO3− ions exit from PCT cells. HCO3− ions are transported out of PCT cells at the basolateral membrane via a sodium coupled, electrogenic bicarbonate cotransporter, NBCe1. This transporter permits an ion complex of one Na+ion and the equivalent of three HCO3− ions to exit as a divalent2−anion, Na(HCO3− )3 .Regulation of proximal tubular reabsorption of bicarbonate ionsLuminal HCO3− ion concentrationHCO3− reabsorption is increased as luminal HCO3− ion concentrationrises because there are more H+ ion acceptors in the luminal fluid. Theopposite is also true: HCO3− ion reabsorption is decreased with a fallin luminal HCO3− ion concentration.

Luminal H+ ion concentrationA higher concentration of H+ ions in the lumen of the PCT inhibitsH+ ion secretion. This scenario occurs, for example, when a patientis given acetazolamide, a drug that inhibits luminal carbonic anhydrase. In this setting, H+ ion secretion is diminished because of therise in the concentration of carbonic acid (H2CO3) and thereby ofH+ ions in the lumen; hence, a smaller amount of filtered HCO3−ions is reclaimed.

1 : principles of acid–base physiology

17

Concentration of H+ ions in PCT cellsA rise in the concentration of H+ ions in PCT cells stimulates thesecretion of H+ ions because of the binding of H+ ions to a modifiersite on NHE-3, which activates this cation exchanger. Intracellularacidosis also increases NBCe1 activity. These effects, however, arenot very important during metabolic acidosis because of the smallerfiltered load of HCO3− ions.Changes in intracellular H+ ion concentration may explain theeffect of K+ ions to modulate the reabsorption of HCO3− ions inthe PCT. Hypokalemia is associated with intracellular acidosis,enhanced reabsorption of HCO3− ions (together with stimulationof ammoniagenesis), and the development of metabolic alkalosis.In contrast, hyperkalemia is associated with a fall in H+ ion concentration in PCT cells, diminished reabsorption of HCO3− ions(together with decreased ammoniagenesis), and the developmentof hyperchloremic metabolic acidosis.Peritubular HCO3− ion concentrationAn increase in the peritubular concentration HCO3− ions decreasesHCO3− ion reabsorption in the PCT.Peritubular PCO2A high peritubular PCO2 stimulates the reabsorption of NaHCO3by the PCT (see margin note). The PHCO3 is elevated in patients withchronic respiratory acidosis.Angiotensin IIAngiotensin II, which is released in response to a decreased EABV,is the most important regulator of reabsorption of NaHCO3 in thePCT. Angiotensin II stimulates NaHCO3 reabsorption by activatingprotein kinase C, which in turn phosphorylates and activates NHE-3.As discussed in Chapter 10, activating NHE-3 and the reabsorptionof NaHCO3 in the PCT leads to an increased reabsorption of NaClin this nephron segment.Parathyroid hormoneActing through adenylyl cyclase and production of cyclic adenosinemonophosphate (cAMP), parathyroid hormone has a small effect toinhibit the reabsorption of HCO3− ions in the PCT.To illustrate the interplay of the different factors that affect reabsorption of HCO3− ions in PCT, consider this example of a patientwho was given a diuretic and developed hypokalemia. PHCO3 willrise initially because of a decreased ECF volume (contraction alkalosis). In addition, hypokalemia is associated with intracellular acidosis, which stimulates ammoniagenesis and hence the additionof new HCO3− ions to the ECF compartment. Factors that stimulate the reabsorption of HCO3− ions in the PCT include a higherluminal concentration of HCO3− ions, the effect of angiotensin II(released in response to a lower EABV) to activate NHE-3, the effectof intracellular acidosis (associated with hypokalemia) to activateNHE-3 and NBCe1, and the higher peritubular PCO2 (metabolicalkalemia suppresses ventilation, leading to a compensatory rise inthe arterial PCO2). On the other hand, the rise in the peritubular

EFFECT OF PERITUBULARHCO3− ION CONCENTRATION,PERITUBULAR PCO2•To separate the effects of peritubular HCO3− ion concentrationversus peritubular PCO2 and/or peritubular pH on HCO3− ionreabsorption in PCT, a techniqueto generate “out of equilibrium”HCO3− ion solution was developed.Its premise is that the reaction ofCO2 and H2O to generate H2CO3occurs relatively slowly, whereasthe reaction of dissociation ofH2CO3 to H+ ions and HCO3− ionsoccurs rapidly. Hence, methodswere developed to rapidly mix twosolutions with different compositions and use the resulting solutionbefore it comes to equilibrium.•Studies using this techniqueshowed that altering the peritubular pH at fixed HCO3− and PCO2did not change the reabsorptionof HCO3− ions by the PCT. It wasthen suggested that the basolateral membrane contains proteinsthat function as HCO3− ion and/or CO2 sensors to mediate theeffects of peritubular HCO3− andperitubular PCO2 on HCO3− ionreabsorption in the PCT.